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—,i S E C0 R 7 0 CONCLUSIONS This Site Conceptual Model was meant to summarize site activities and provide a guide for 1 future assessment and remediation Figure 20 presents a graphical interpretation of the general conceptual site model The following conclusions result from the above collection of data • Petroleum hydrocarbons detected in soil and groundwater during the removal of USTs, product lines, and dispensers in 1999 and subsequent site investigation resulted from operations of gasoline USTs and associated piping and dispenser at the site The ' majority of hydrocarbons in soil are in the area of the former dispensers to depths of 80 feet bgs x • Well surveys for the site and an adjacent LUFT site indicated that the nearest supply ' wells are approximately 1,500 feet northeast (well could not be field verified and may not „ exist) and 2,000 feet east of the site at the Stockton State Hospital • The vertical extent of benzene, MtBE, and 1,2-DCA in soil has been delineated to . nondetect levels and low levels of TPHg ' ,�Ss • Monitoring and sampling performed at the site since 2000 indicates the presence of ' TPHg, benzene, MtBE, and 1,2-DCA dissolved in groundwater beneath the site with a r majority of the dissolved mass in the B monitoring zone The extent of TPHg, benzene, ' and MtBE in groundwater cross and downgradient of the site are defined in the B r . monitoring zone 1,2-DCA is not delineated in the B monitoring zone • TPHg, benzene, MtBE, and 1,2-DCA impacts to groundwater in the A monitoring zone at N the Former 76 Station appear to be affected by upgradient ARCO service station 4932 ' • Initial groundwater analytical results for samples collected from the C monitoring zone wells indicate elevated levels of TPHg (reported as GRO), benzene, and 1,2-DCA Exposure to contaminants associated with petroleum hydrocarbons adsorbed to soil and ;¢, dissolved in groundwater at the site is depicted in Figure 20 Potential exposure medium include the following groundwater, soil, and air Groundwater is typically only encountered ' during monitoring well gauging, groundwater sampling, and well installation Soil is typically only encountered during well installation or on-site excavation Despite the presence of asphalt at the site, air (ambient) may be encountered via cracks in the asphalt surfaces should the site be ' redeveloped Figure 21 presents an exposure pathway model, depicting sources responsible for potential exposure and potential pathways of exposure x � Based on the above information SECOR would recommend the following _ The site should continue to be sampled on a quarterly basis to confirm groundwater ; concentrations and to determine plume stability in the A, B, and C zones n' Isoconcentration maps for TPHg, benzene, MtBE, and 1,2-DCA in all three zones should be included in future quarterly monitoring and sampling reportsw MW-15, MW-16 and MW-17 will be added the current quarterly monitoring and sampling ' program SECOR recommends the wells be monitored and sampled for at least three quarters in order to determine the validity of the initial analyses Any recommendations for additional assessment will be considered at that time Updated SCM 2005 doc 17 A ' 77CP 64409 05 0005 �m